The present invention relates to techniques for manufacturing magnetic disks employed in data storage applications, to disks formed as described herein, and disk drives incorporating such disks. More particularly, the present invention relates to magnetic disks and manufacturing techniques therefor which improve corrosion resistance and magnetic performance.
The use of a substrate such as glass, or aluminum covered with plated nickel phosphorus (NiP), as a base to manufacture magnetic disks for data storage is well known. For purposes of discussion, FIG. 1 shows an exemplary aluminum alloy substrate 102. Above substrate 102, there is disposed a layer of nickel phosphorus 104, typically formed by a deposition process such as electroless plating. Typically, the NiP layer is plated to a thickness of several microns. This thickness is required because a substantial amount is subsequently polished off to create a smooth surface. The polishing is relatively extensive because the as-deposited surface of the NiP layer is rough, which in turn is due to the relatively rough surface of the aluminum substrate. A high thickness of NiP is also required to provide a hard surface compared to that of the soft aluminum substrate, to reduce the damage caused by sudden head impact (xe2x80x9cdingingxe2x80x9d) during operation of the disk drive.
There are typically additional layers disposed above NiP layer 104, such as an underlayer typically comprising chromium (Cr) (as used herein, Cr or a layer of Cr shall be understood to include Cr alloys), an overlayer of magnetic material (such as a cobalt alloy or iron alloy) disposed above the Cr underlayer, and a protective overcoat.
By way of background, the NiP layer is typically textured to provide a preferential degree of orientation of magnetic moments in the overlying magnetic layer. Generally, the NiP layer is textured by forming texture grooves in the downtrack direction. As the term is employed herein, the downtrack direction shall be understood to be generally orthogonal or near orthogonal to the radial direction of the disk and may include concentric, crosshatch, or at times non-parallel patterns. The texture grooves cause a preferential alignment of magnetic moments along the downtrack direction in the cobalt alloy layer.
As is well known to those skilled in the art, this preferential alignment of magnetic moments allows for increased coercivity and hysteresis squareness in the downtrack direction which makes it possible to reliably store bits of data at high density in the magnetic layer as compared with an isotropic layer. The high squareness is important because it results in a higher magnetic remanence (Mr) in the downtrack direction. As is known, the signal strength is proportional to Mr times the thickness (T) of the magnetic layer, or MrT. While it is desirable to have a high MrT for the signal, it is also imperative to reduce the effective space loss between the read/write element and the magnetic layer to 1 microinch (xcexcxe2x80x3) or lower. The effective space loss is the distance between approximately the center of the magnetic layer and the read and write element. Thus, there has been a continuing trend towards reduced magnetic layer thickness, T. With the higher Mr provided by preferential orientation therefore, a lower thickness magnetic layer can be employed while still maintaining sufficient MrT. In addition to reducing the thickness of the magnetic layer, other methods to reduce the space loss include reducing the thickness of the protective overcoat layer, and reducing the head-media spacing during read and write operations.
Returning to the texture process, in the current art, the NiP layer is typically textured using a mechanical abrasion process. In one case, the mechanical abrasion process essentially abrades the NiP layer along the downtrack direction using a tape having thereon abrasive particles. Unfortunately, it has been found that the mechanical abrasion process tends to gouge the NiP layer forming some grooves that are excessively deep. Additionally, high ridges are formed along the gouged groove.
FIG. 2 is an atomic force microscope (AFM) scan of a textured NiP layer. It will be appreciated that the horizontal and vertical scales of FIG. 2 are not the same as one another. As can be seen, some grooves, such as groove 202, are excessively deep and narrow, while others are of approximately the desired depth for inducing the preferred magnetic orientation. In addition, along deep groove 202 is ridge 203, which is higher than desired. Although the non-uniformity among groves can be minimized by using abrasive slurries having a more uniform distribution of particles and by controlling the abrasion process more precisely, the very mechanical nature of the mechanical abrasion process renders it impossible to eliminate the nonuniformity completely.
FIG. 3A illustrates a problem that occurs with deep gouges. It will be appreciated that the drawings of the grooves such as that shown in FIG. 3A are not necessarily to scale. In FIG. 3A, layer 306 represents the overcoat layer. It will be appreciated that there are other layers, not shown in FIG. 3A, underlying layer 306. Such layers may include, for example, one or more underlayers, one or more magnetic layers, one or more overcoat layers, and one or more additional layers that may be deposited by e.g. sputter deposition. The overcoat layer 306 typically comprises a carbon-containing layer. As layer 306 is deposited, the depth and profile of deep groove 302 makes it difficult for layer 306 to adequately cover the NiP surface. As a result, voids or gaps in layer 306 may be present near the vicinity of deep groove 302. The layers underlying layer 306 may or may not have voids in deep grooves, depending on their thickness and other factors. As a result of the gaps, one or more of the various layers under layer 306, and/or the NiP layer and/or the substrate are now exposed to moisture, which causes corrosion. The Co alloy layer is particularly susceptible to corrosion, and is the primary cause of concern. Additionally, the other layers, and the substrate material are also susceptible to corrosion to varying degrees. In any event, corrosion will lead to the generation of particles that are picked up by the head resulting in degraded drive performance
In contrast to groove 302 of FIG. 3A, groove 304 is of about the desired profile, and layer 306 can cover the entire surface in the region of groove 304. Because layer 306 is a good moisture barrier, corrosion is prevented because moisture can not penetrate to the layers underneath layer 306. The formation of gaps in layer 306 becomes more likely as the thickness of layer 306 is reduced, so that the problem depicted in FIG. 3A can be expected to get worse in future products.
As mentioned above, a further problem that may occur during texturing is the formation of ridges, such as ridge 305 in FIG. 3A, along the gouged grooves. While the deposition coverage of the various layers over high points is generally good, there may be a failure to cover extremely sharp points, particularly by the thin protective overcoat layer, so that underlying layers are exposed and therefore susceptible to corrosion. An additional concern arises with respect to ridges sufficiently high to collide with magnetoresistive heads, giant magnetoresistive heads and the like, because such collisions cause a temperature rise of the magnetoresistive element, which generates a false signal. This failure mechanism is referred to as thermal asperity. Because of this, as one of the later stages of manufacture, after all layers have been deposited, a burnish step is performed which effectively knocks off any high points. When the asperity is knocked off during burnish, a portion of layer 306 is knocked off as well. This problem is also particularly severe with overcoat layers having a low thickness. In any event, because of this one or more layers, such as the Co alloy layer, will be exposed, leading to corrosion at that site.
To determine corrosion susceptibility, a disk is subjected to either a high temperature, high humidity environment, or is subjected to a hydrochloric acid (HCl) test. The disk is then examined under a dark field optical microscope. FIG. 3B shows a photomicrograph of corrosion sites on a disk, which appear as light areas or spots on a dark background. As can be seen, many of the corrosion sites occur along a line. This is due to the above-described ridges or gouges in some of the texture lines.
Glass substrates pose particular challenges. It is difficult to texture glass substrates because of their hardness and because texturing can cause microscopic fractures along the texture lines. Moreover, it has been found that textured glass does not induce a preferred orientation in the manner that a textured NiP layer does. Because glass is non-metallic, it cannot be plated with a NiP layer in a conventional manner. A NiP layer can be deposited by vacuum techniques, such as described in U.S. Pat. No. 5,250,339. However, it is asserted therein that such films must be sufficiently thick to prevent the NiP film from peeling off and to provide a uniform surface. Although the ""339 patent describes a lower limit of 0.03 micron for the NiP layer, it can be seen from the data therein that a uniform surface finish is not obtained until a NiP thickness of approximately 0.1 micron.
Additionally, because glass is a poor thermal conductor, the thermal asperity problem is exacerbated. Moreover, as fly heights are reduced, occasional contact between the media and the head occurs. This causes a local rise in the media temperature, which, if not uniformly dissipated, causes a baseline shift in the signal. Because of the foregoing, it is known to be desirable to put a thermally conductive layer, such as a vacuum deposited metal layer, or thick electroless NiP layer (after first forming an underlayer to enable plating on the glass substrate) below the various media layers. Depending upon the thermal conductivity of the material, such layer typically must be about 0.1 micron or greater. In addition, the layer must be sufficiently thick to eliminate any possibility of the texturing process reaching the glass substrate, as the slurry will scratch the substrate surface, generating defects. One problem with thick NiP layers is the formation of deep gouges and high ridges as described herein. Another problem with increased layer thickness is that the internal stress increases, so that delamination of the layer may occur. A further problem with high thickness layers formed by vacuum deposition is the relatively high cost.
U.S. Pat. Nos. 5,681,635 and 5,855,951 propose use of a hard film on glass ceramic substrate. The films described therein are selected on the basis of their ability to form a pseudo-diffusion region and a graded interfacial region between the layer and the glass ceramic substrate. Specifically, the deposited materials interact with the ceramic substrate material to form a compressive stress, which increases the substrate""s strength. On this layer, a softer, texturable layer is deposited. The exemplary material described is a titanium nitride (TiN) layer as the hard underlayer followed by a softer titanium (Ti) rich TiN layer. Unfortunately, the layers described in these patents are not practical for several reasons. First, some of the materials described are crystalline in nature. Such materials form crystal facets at sufficiently great thicknesses, which increases the surface roughness. For example, the Ra roughness of an exemplary film is stated to be less than one microinch (xcexcxe2x80x3). The required Ra roughness of current and future disks is well below one xcexcxe2x80x3 such that surfaces with a roughness in this range are unacceptable. In addition, the materials described in the foregoing patents are not well known or characterized in disk drive applications, particularly the suggested Ti rich TiN texture layer, in which it may be very difficult to create a desirable texture structure. The thicknesses of 0.5-5.0 microns (xcexcm) for the lower layer and 100-150 nanometers (nm) (i.e. 0.1-0.15 xcexcm) for the texture layer are relatively high, resulting in increased costs.
In view of the foregoing, there are desired improved techniques and structures for improving magnetic performance in a magnetic media that employs grooves formed in the downtrack direction in a texture layer such as a NiP layer. Preferably, such media should have one or more of the following characteristics: It should be compatible with the requirements of very low fly height (contact or near contact) recording. As the head flies closer to the surface of the disk, the glide requirements will be more stringent, resulting in a greater number of corrosion sites unless high points are substantially reduced prior to burnish. Moreover, media should also be able to tolerate the continually decreasing overcoat thicknesses needed to meet the demands of future high density media. Any void in the overcoat layer results in a corrosion site, and this problem can be expected to get worse as overcoat layers get thinner. Currently, the state of the art demands overcoat thickness as low as approximately 50 xc3x85, with lower thicknesses to be employed in the future as the areal density continues to increase.
Further, any layer used for texturing should preferably work at a relatively low thickness, particularly if it is to be used with glass substrates. This is important because glass presents a smooth surface, which is needed for low fly height, and a very hard surface, which is resistant to defects such as embedded particles. A low thickness texture layer essentially preserves the advantages of the glass layer. A further preferable feature, particularly with glass, is that the material should have good thermal conductivity to dissipate heat, to mitigate the effects of local temperature rise resulting from occasional head-media contact during read operations. It is also desirable for the layer to have good electrical conductivity as well, so that a bias can be applied during sputtering on non-conductive substrates. The layer should also have good adhesion to a glass substrate to prevent delamination or to avoid a costly additional adhesion layer. In addition, as vacuum deposition is an expensive step, low layer thickness is desirable to achieve lower costs. Moreover, any metallization process and structure for glass, glass ceramic and similar substrates should be relatively simple and low cost, because these substrates are typically considerably more expensive than conventional aluminum substrates.
It is also preferable that the texture layer be amorphous, as crystalline layers tend to form with facets, increasing the roughness of the surface. It is also preferable that the texture layer comprise a well characterized and understood material such as nickel phosphorus (NiP), so that the design of the media can proceed with the advantage of these known characteristics, including its effect on the magnetic properties of the media. The material of the layer should itself exhibit good corrosion resistance. In this regard, an amorphous structure is further desirable because amorphous layers generally have superior corrosion resistance (even as compared to the same material in crystalline form) because of the lack of grain boundaries that usually accelerate corrosion.
The invention relates, in one embodiment, to a method for improving corrosion resistance while maximizing magnetic performance of a magnetic disk employed in hard disk drive applications. The invention includes providing a substrate and forming a first layer above the substrate, the first layer having a first degree of abrasion resistance. The invention includes forming a second layer above the first layer, the second layer having a second degree of abrasion resistance lower than the first degree of abrasion resistance. The invention further includes forming downtrack grooves in the second layer.
In another embodiment, the invention relates to a magnetic disk for data storage, which includes a substrate, and a first layer disposed above the substrate, the first layer having a first degree of abrasion resistance. The magnetic disk further includes a second layer disposed above the first layer, the second layer having a second degree of abrasion resistance, the first degree of abrasion resistance being higher than the second degree of abrasion resistance, the second layer having thereon downtrack grooves.
In yet another embodiment, the invention relates to a magnetic disk drive for storing data, which includes a magnetic disk including a substrate, a first layer disposed above the substrate, the first layer having a first degree of abrasion resistance, and a second layer disposed above the first layer, the second layer having a second degree of abrasion resistance, the first degree of abrasion resistance being higher than the second degree of abrasion resistance, the second layer having thereon downtrack grooves, and a magnetic layer disposed above the second layer. The magnetic disk drive further includes a motor coupled to the magnetic disk for rotating the magnetic disk, and a read-write head configured to be disposed in a spaced-apart relationship with the magnetic layer for reading data from and writing data to the magnetic disk. Additionally, there is included an arm for holding the read-write head in proximity to the magnetic disk, and an actuator for moving the arm so that the read-write head may be placed over desired positions on the magnetic disk.
Embodiments of the present invention preferably have one or more advantageous features including use of an amorphous layer for the first layer, use of nickel niobium (NiNb) for the first layer, use of an amorphous, non-magnetic nickel-containing layer, such as NiP, for the second layer, low as-formed surface roughness of the first and second layer, and relatively low thickness of the second layer and of both layers combined.
These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.